The invention relates to a process for the formation of vulcanized hollow bodies, and the hollow bodies so obtained. More particularly, the vulcanization of components from natural or synthetic rubber or elastomers, combined with each other and/or different materials such as leather, skin, cotton and/or metals, expanded rubbers and other materials takes place on the inside of heated molds. A pressurized fluid, such as hot or cold air, vapor or compatible gas is injected between said hollow bodies being formed. During vulcanization, the peripheral components, coupled with each other with airtightness along at least a common contact line, are pushed and kept in forming adhesion against the mold peripheral surfaces by the pressurized fluid. The process is carried out at temperatures preferably between about 100xc2x0 C. and about 170xc2x0 C., and at maximum pressures of the forming fluids of some bars, and is adoptable for the formation of any vulcanizable hollow components, such as for instance: tires and the like, seats, cushions, mattresses, products with sealed and independent zones, with different hardness, pressure and/or thickness, products for footwear in general, such as sandals, normal shoes, shoes with peripheral borders from rubber or other vulcanizable material, boots, slippers.
The process adopted by the known art for the production of vulcanized items, such as the parts of shoes and the like, are rather limiting. In fact, they allow to obtain, for instance, soles and/or arch-supports separate from each other, but not to obtain a combined formation of soles having intermediate chambers for air circulation. More precisely, in said vulcanization processes, the desired rubber composition obtained with a closed mixer, passes to an open mixer wherefrom the strip-cut sheets are conveyed to preforming machines that prepare the rubber pieces or the like in the desired size. The pieces are then arranged in the heated molds wherein they take on by means of presses the desired form by a vulcanization process. Rubber must remain in the molds for a given time, according to the type of rubber composition utilized and the thickness of the resulting products. The products obtained are single and compact, without the possibility of forming in them airtight- or air circulation chambers. Other problems arise from the fact that said processes require the use of large and very expensive presses, and the use of expensive steel molds for the forming process. The high cost of the steel molds is due, in particular, to the number of molds necessary to form, for instance a complete work set. Since each mold can be used only for an individual forming, with the ensuing need of having at hand as many molds as there are types of forming, design and thickness necessary for production. For other particular processes, such as for instance those relating to the application by vulcanization of peripheral borders on vamps already mounted on the respective shoe-trees and with the soles pre-assembled and already glued, it is at present necessary to have recourse to the use of an auto-clave, with consequent high plant and management costs.
An object of the present invention relates to a solution that eliminates the above drawbacks by adopting a process for the formation of vulcanized hollow bodies, and the hollow bodies obtained by such process, in order to achieve very significant results and advantages. More precisely, on the inside of the hollow bodies, zones, one or more chambers of any shape or development, airtight, independent or interconnected, subject to equal or differential pressures, are obtainable. The bodies may be circumscribed with materials having the same or a different firmness or hardness, according to specific local conditions. The vulcanization of the components, from natural or synthetic rubber or treated elastomers that may be combined with different materials such as leather, skin, cotton, metals, expanded rubbers or other compatible materials, is obtained in molds heated at temperatures comprised, for instance, between about 100xc2x0 C. and about 170xc2x0 C., wherein the forming element for obtaining the desired detail is a fluid, such as hot or cold air, vapor or other suitable gas, at an elevated pressure, for instance 3 to 6 bars, that is injected on the inside of said components intended for forming the hollow bodies. With said process it is possible to produce composite hollow bodies of any shape, with one or more internal chambers having different configurations and extensions and subject to different pressures, independently on the adopted and combined materials and the use requirements of said hollow bodies. Vulcanization does not require presses, as the molds, formed from half-molds closed and blocked with one another after the positioning therein of the pieces to be composed and vulcanized, can be made of a material sufficient to resist the closing loads and the pressure of the fluid injected inside, and capable of supporting contained temperatures, comprised for instance between about 100xc2x0 C. and about 170xc2x0 C., with thermal jumps between room temperature and maximum temperature, and vice-versa, obtained in sufficiently rapid times. The production cycle adopted for the process and the means necessary, such as molds, fluid injectors, injection valves, hot or cold air tunnel conveyors, are of a contained and economical cost.
The process also permits the forming of composite hollow bodies, suitably sized for special applications, with reduced working times and costs, and with a high yield. In particular and for instance, in the footwear industry, the simultaneous formation of soles having incorporated arch-supports, soles with air cushion and independent circulation arch-supports, of leather and/or cloth upper attachments, connections with stiffeners, of soles either made entirely from rubber or with leather insoles and heels, and other possibilities can be accomplished. The same system can be used to produce slippers, multi-color soles with or without internal air circulation or with discharge towards the outside, and also shoes with peripheral borders of rubber or other vulcanizable materials.
The technology characteristic of the present invention allows to obtain high reliability products with a versatile and economical process that ensures a reduction in machine and mold costs.